It is known to the art that helicopters may be classified into three general types having:
A. Rotor with articulated hub;
B. Rotor with semi-rigid hub.
C. Rotor with rigid hub.
To the first type (a) pertain rotors the blades of which can perform three classes of movement, around hinges viz: "lag", "flapping", and "pitch variation"; or "feathering"in the second type of rotor (b) the blades can perform the second and third types of movement while in rotors of the third type (c) the blades can perform pitch variation only.